This invention relates generally to fuel cell systems, and more particularly to an apparatus and method for recycling hydrogen fuel gas to a fuel cell stack.
A fuel cell is a device that converts chemical energy directly into electrical energy and heat. In perhaps its simplest form, a fuel cell comprises two electrodesxe2x80x94an anode and a cathodexe2x80x94separated by an electrolyte. During use, the anode is supplied with fuel and the cathode is supplied with an oxidizer, which is usually oxygen in ambient air. With the aid of a catalyst, the fuel undergoes oxidation at the anode, producing protons and electrons. The protons diffuse through the electrolyte to the cathode where, in the presence of a second catalyst, they combine with oxygen and electrons to produce water and heat. Because the electrolyte acts as a barrier to electron flow, the electrons travel from the anode to the cathode via an external circuit containing an electrical load that consumes power generated by the fuel cell. A fuel cell generates an electrical potential of about one volt or less, so individual fuel cells are xe2x80x9cstackedxe2x80x9d in series to achieve a requisite voltage.
Because of their high efficiency, their potential for fuel flexibility, and their ability to generate electricity with zero or near zero emission of pollutants, fuel cells have been proposed as replacements for internal combustion engines in vehicles. Among fuels that have been considered for vehicle applications, hydrogen (H2) appears to be the most attractive. Hydrogen has excellent electrochemical reactivity, provides sufficient power density levels in an air-oxidized system, and produces only water upon oxidation.
FIG. 1 schematically shows a hydrogen-based fuel cell system 10. The fuel cell system 10 includes a fuel cell stack 12, which is made up of individual fuel cells 14 and includes cathode 16 and anode 18 terminals that are electrically connected via an external circuit 20. The external circuit 20 includes a load 22 (e.g., electrical motor) which consumes power generated by the fuel cell stack 12. Air (oxygen) and pressurized hydrogen enter the fuel cell stack 12 through cathode 24 and anode 26 gas inlets, respectively. The fuel cell stack 12 includes internal flow paths 28, 30, which distribute air and hydrogen to the cathode and anode of each fuel cell 14. Oxygen-depleted air exits the fuel cell stack 12 through a cathode gas outlet 32. Water, nitrogen, and unreacted hydrogen exit the fuel stack 12 through an anode gas outlet 34.
As shown in FIG. 1, a first conduit 36 carries the anode gases (H2, N2, and H2O) away from the fuel cell stack 12. A portion of the anode gas stream may vent into an exhaust line 38 through a draw-off valve 40; a recycle line 42 returns the balance of the anode gas stream to the fuel cell stack 12. Besides pressure losses from anode gas venting, frictional losses within the anode gas flow path 30 typically result in about a thirty kPa pressure drop across the fuel cell stack 12. To overcome these pressure losses, the fuel cell system 10 employs a motor 44 driven blower 46 to boost the pressure of the anode gas within the recycle line 42. For clarity, the motor 44 and blower 46 are depicted without an enclosure to show that a rigid shaft 48 transmits torque between the motor 44 and blower 46. Furthermore, as indicated by an arrow 50, a dynamic seal 52 reduces, but may not eliminate the flow of the anode gas from the blower 46 to the motor 44.
Pressurized anode re-circulation gas exits the blower 46 through an outlet 54 and flows into a discharge line 56, which directs the anode gas recycle stream into the anode gas inlet 26 of the fuel cell stack 12. A second conduit 58, which communicates with a hydrogen gas reservoir 60 or other source of hydrogen, introduces make-up hydrogen into the blower discharge line 56. A control valve 62 and a mass flow meter 64, which communicate with a flow controller (not shown), regulate the amount of hydrogen added to the anode gas re-circulation stream. During operation, a heat exchanger 66 removes excess heat generated by the blower motor 44. The heat exchanger 66 typically comprises a fluid coolant loop 68, which circulates the fluid coolant through the motor 44 housing.
Although the fuel cell system 10 shown in FIG. 1 represents a useful scheme, existing motor 44 driven blowers 46 for fuel cell applications present several difficulties. Because hydrogen is a small molecule, the dynamic seal 52 may be unable to completely prevent H2 from leaking into the blower motor 44 air space. In addition, water in the anode gas re-circulation stream may leak into the motor 44 housing, which may contaminate the motor lubricant and promote corrosion of motor parts. Finally, since the motor 44 generates a substantial amount of heat, a relatively large heat exchanger 66 must be used, which adds to the bulk and expense of the fuel cell system 10.
The present invention overcomes, or at least helps mitigate, one or more of the problems set forth above.
The present invention provides a fuel cell system that can be used to power a vehicle. The system includes a fuel cell stack, which uses hydrogen and an oxidizer (typically oxygen in ambient air) to generate electricity. The system includes a re-circulation loop for returning unreacted hydrogen, along with water and nitrogen, to the fuel cell stack, and a hermetically sealed assembly, which comprises a blower portion for pressurizing hydrogen in the re-circulation loop and a motor portion for driving the blower.
The system also includes a source of make-up hydrogen for replenishing hydrogen in the re-circulation loop. The source introduces make-up hydrogen in the motor portion of the assembly at a pressure greater than the pressure in the blower portion of the assembly. As a result, at least some of the make-up hydrogen flows from the motor portion of the assembly into the blower portion assembly, which helps prevent components in the re-circulation loop from entering the motor portion of the assembly. Make-up hydrogen purges the motor of undesirable compounds (e.g., water and oxygen) and removes heat generated by the motor and controller (if present). Passing make-up hydrogen through the blower portion of the assembly preheats the make-up hydrogen and, in some cases, obviates the need for a separate heat exchanger.
The present invention also provides an apparatus for replenishing hydrogen in a fuel cell stack. The apparatus includes a re-circulation loop for returning unreacted hydrogen to the fuel cell stack, and a hermetically sealed assembly comprising a blower portion and a motor portion. The blower portion of the assembly, which communicates with the re-circulation loop, pressurizes hydrogen in the re-circulation loop, and the motor portion of the assembly drives the blower. The apparatus includes a source of make-up hydrogen, which is adapted to introduce hydrogen in the motor portion of the assembly at a pressure greater than the pressure in the blower portion of the assembly.
Finally, the present invention provides a method of replenishing hydrogen in a fuel cell stack. The method comprises re-circulating unreacted hydrogen from an outlet to an inlet of the fuel cell stack using a motor-driven blower. The motor, which is hermetically coupled to the blower, has a flow path that provides fluid communication between the motor and the blower. The method thus includes introducing make-up hydrogen in the motor at a pressure higher than the pressure in the blower. Make-up hydrogen flows within the motor and through the flow path into the blower where it mixes with unreacted hydrogen.